1. Field of the Invention
This invention relates to a novel catalytic cracking process to produce motor fuels. In particular, this invention relates to an improved catalytic cracking process for producing motor fuel involving the use of a catalyst such as large crystallite ZSM-5 in conjunction with a conventional zeolite-containing cracking catalyst to thereby increase gasoline octane number and gasoline plus alkylate yield.
2. Description of the Prior Art
Hydrocarbon conversion processes utilizing crystalline zeolites have been the subject of extensive investigation during recent years, as is obvious from both the patent and scientific literature. Crystalline zeolites have been found to be particularly effective for a wide variety of hydrocarbon conversion processes including the catalytic cracking of a gas oil to produce motor fuels and have been described and claimed in many patents, including U.S. Pat. Nos. 3,140,249; 3,140,251; 3,140,252; 3,140,253; and 3,271,418. It is also known in the prior art to incorporate the crystalline zeolite into a mixture for catalytic cracking and such disclosure appears in one or more of the above-identified U.S. patents.
It is also known that improved results will be obtained with regard to the catalytic cracking of gas oils if a crystalline zeolite having a pore size of less than 7 Angstrom units is included with a crystalline zeolite having a pore size greater than 8 Angstrom units, either with or without a matrix. A disclosure of this type is found in U.S. Pat. No. 3,769,202. Although the incorporation of a crystalline zeolite having a pore size of less than 7 Angstrom units into a catalyst composite comprising a large pore size crystalline zeolite (pore size greater than 8 Angstrom units) has indeed been very effective with respect to raising of octane number, nevertheless, it did so at the expense of the overall yield of gasoline.
Improved results in catalytic cracking with respect to both octane number and overall yield were achieved in U.S. Pat. No. 3,758,403. In said patent, the cracking catalyst was comprised of a large pore size crystalline zeolite (pore size greater than 7 Angstrom units) in admixture with ZSM-5 type zeolite wherein the ratio of ZSM-5 type zeolite to large poe size crystalline zeolite was in the range of 1:10 to 3:1. Effective cracking processes were disclosed as being achieved when the catalyst was used to obtain the inherent advantages realized in moving bed techniques, such as the Thermofor catalytic cracking process (TCC) as well as in fluidized cracking processes (FCC).
U.S. Pat. No. 3,758,403 is silent as to the crystallite size of the ZSM-5 employed. It does mention at Column 3, lines 25-31, that ZSM-5 is disclosed in application Ser. No. 865,472, filed Oct. 10, 1969, which is now U.S. Pat. No. 3,702,886. U.S. Pat. No. 3,702,886 contains examples which are silent as to crystalline size, e.g., Example 1. It also contains examples wherein the crystallite size of ZSM-5 is on the order of 1 micron, e.g., Example 2; examples wherein the crystallite size is less than 1 micron (Example 26) and Examples wherein the ZSM-5 has a crystallite size of 8 X 20 microns and "some large cubes to 25 microns" (Example 27).
U.S. Pat. No. 3,758,403 does contain a disclosure as to particle size of "each type of zeolite making up the catalyst system" (Col. 9, lines 25-31) but particle size and crystalline size are separate and distinct properties--as will be further explained.
The use of ZSM-5 type zeolites in conjunction with a zeolite cracking catalyst of the X or Y faujasite variety is described in U.S. Pat. Nos. 3,894,931; 3,894,933; and 3,894,934. The two former patents disclose the use of ZSM-5 type zeolite in amounts up to and about 5 to 10 weight percent; the latter patent discloses the weight ratio of ZSM-5 type zeolite to large pore size crystalline zeolite within the range of 1:10 to 3:1.
The addition of a separate additive catalyst comprising one or more members of the ZSM-5 type has been found to be extremely efficient as an octane and total yield improves when used in very small amounts in conjunction with a conventional cracking catalyst. Thus, in U.S. Pat. No. 4,309,279, it was found that only 0.1 to 0.5 weight percent of a ZSM-5 type catalyst added to a conventional cracking under conventional cracking operations could increase octane by about 1 to 3 RON+O (research octane number without lead).
Said U.S. Pat. No. 4,309,279 is silent as to the crystalline size of the ZSM-5 used.
U.S. Pat. No. 4,309,280 also teaches ZSM-5 and other zeolites in conjunction with a conventional cracking catalyst. The ZSM-5 is stated in Examples 10-13 to have a crystalline size of 0.02 to 0.05 microns.
Prior to the instant invention it had always been believed that smaller crystallite size ZSM-5 was better than a larger crystallite size material because of the fact that the smaller crystallite size aged more slowly. This is reflected in patents such as U.S. Pat. Nos. 3,761,226 and 3,926,782.
U.S. Pat. No. 3,781,226 discloses at Column 1, lines 43-47 that "numerous material and synthetic zeolites, especially crystalline aliminosilicate zeolites, have been used for various organic compound conversion reactions. These natural or synthetic zeolites generally exist in a particle size range above 1 micron." Said patent points out the improved benefits resulting from using zeolites, including ZSM-5, which have particle size of between 0.005 microns and 0.3 microns (Column 4, lines 10-14). Although the patent stresses particle size as opposed to crystallite size; nevertheless, ZSM-5 having a crystallite size of 0.04 micron is disclosed in Example 2.
U.S. Pat. No. 3,926,782 teaches ZSM-5 type crystals for hydrocarbon conversion having an ultimate particle diameter of 0.005 to 0.1 micron as crystallized and specifically points out that these small crystallite materials age at a slower rate in various hydrocarbon conversion processes including catalytic cracking (Column 4, lines 57-65). At Column 6, lines 12-18, the patent states that catalysts made with small size ZSM-5 crystals (0.005-0.1 micron in ultimate particle, i.e., individual crystallite diameter) have been found to age at a significantly slower rate than corresponding ZSM-5 containing catalysts of appreciably larger crystallite size (0.1-0.5 micro in ultimate particle, i.e., individual crystallite diameter).
The difference between crystallite size, i.e., the size of the individual crystal and particle size, i.e., the size of the many crystals which have agglomerated due to surface forces is best illustrated in Example 21 of said patent. In said Example, the ZSM-5 had a particle size of 0.25-0.5 microns in diameter whereas the individual crystallites were about 0.02 microns in diameter. As has heretofore been stated, this invention is concerned with crystalline size, i.e., the size of the individual crystals.
Large crystallite size ZSM-5 type zeolites are known in the art and in this connection U.S. Pat. No. 4,375,458 teaches materials having a crystal size at least 1 micron. U.S. Pat. No. 4,100,262 teaches ZSM-5 crystals having a size between 5.times.10 microns and 10.times.20 microns including their use in cracking. U.S. Pat. No. 4,117,026 discloses crystalline aluminosilicate zeolites including ZSM-5 having a crystallite size of greater than about 0.5 microns generally in the approximate range of 1-20 microns and particularly 1-6 microns, i.e., Column 4, lines 42-46. However, the patent is silent as to the use of these materials for catalytic cracking of gas oils in order to produce hydrocarbons boiling in the motor fuel range.
In order to reduce automobile exhaust emissions to meet federal and state pollution requirements, many automobile manufacturers have equipped the exhaust system of their vehicles with catalytic converters. Said converters contain catalysts which are poisoned by tetraethyl lead. Since tetraethyl lead has been widely used to boost the octane number of gasoline, refiners now have to turn to alternate means to improve gasoline octane number.
One method of increasing octane numbers is to raise the cracker reactor temperature. This method, however, is very limited, since many units are now operating at maximum temperatures due to metallurgical limitations. Raising the cracker reactor temperature also results in increased requirements for the gas plant (i.e., gas compressor and separator). Since most gas plants are now operating at maximum capacity, any increased load could not be tolerated by the present equipment.
An alternative method has been to mix an additive catalyst such as ZSM-5 to the cracking catalyst as described above. Generally, the octane gain for ZSM-5 containing cracking catalyst is associated with gasoline (C.sub.5 +) yield decrease and correspondingly higher yields of C.sub.3 and C.sub.4 gaseous products.
As can well be appreciated in the foregoing, it would be extremely desirable to have a process which will provide high octane unleaded gasoline without undue sacrifice of gasoline yield.